Add tap test for pg_signal_autovacuum role

Add tap test for pg_signal_autovacuum role

This commit provides testig coverage for ccd38024bc3c, checking that a
role granted pg_signal_autovacuum_worker is able to stop a vacuum
worker.

An injection point with a wait is placed at the beginning of autovacuum
worker startup to make sure that a worker is still alive when sending
and processing the signal sent.

Author: Anthony Leung, Michael Paquier, Kirill Reshke
Reviewed-by: Andrey Borodin, Nathan Bossart
Discussion: https://postgr.es/m/CALdSSPiQPuuQpOkF7x0g2QkA5eE-3xXt7hiJFvShV1bHKDvf8w@mail.gmail.com